function objToString(obj) {
    var str = ''
    for (var key in obj) {
        str += `${key}=${obj[key]}&`
    }
    return str.slice(0, str.length - 1)

}
function ajax(method,url ,mains,Cat){
    var xhr = new XMLHttpRequest()
    xhr.open(method,method == 'get'? url+"?"+objToString(mains) : url)
    xhr.onreadystatechange = function(){
        if(xhr.readyState == 4){
            Cat(JSON.parse(xhr.responseText))
        }
    }
    xhr.setRequestHeader('content-type','application/x-www-form-urlencoded')
    xhr.send(method == 'get'? null :objToString(mains))
}